Submerged combustion is a method of heating whereby hot products of combustion are forced through a medium to heat the medium. The heat exchange occurs directly between the hot products of combustion and the medium, which may be water or an aqueous solution. In a submerged combustion system, the hot combustion products are generated by a flame fed by a combination of air and a suitable fuel. The flame typically does not actually come into contact with the medium. This technology differs from conventional heat exchange methods such as immersion tube heating where the heat exchange is indirect from combustion products to the tube and through the tube walls to the medium. In conventional heat exchange methods, the spent products of combustion are inefficiently exhausted directly to the atmosphere with useable heat remaining, rather than being efficiently exhausted through the medium and giving up all possible heat, as is possible in submerged combustion.
A problem which exists in hydrocarbon recovery operations, such as oil and gas wells is the production of sometimes large quantities of water together with the desired hydrocarbons, which water must be disposed of in an economical and environmentally friendly way. A great variety of apparatuses and methods have been proposed for disposing of large quantities of water that cannot simply be poured on the ground or into a waterway. Generally these methods have included impoundment and evaporation, use for irrigation of crops, and in some cases active heating or conversion to steam. However, none of these methods efficiently combine high rates of heat transfer from the fuel to the water, as needed to achieve the maximum evaporation rate at the maximum fuel efficiency.
Submerged combustion devices have been applied to the problem of heating water, both for production of hot water per se and for other applications such as melting snow. The submerged combustion devices used in prior art systems often employ a vertical combustor/weir style apparatus having a relatively small coverage area compared to the volume of water to be heated, in which the combustion gases are directed downwardly onto the liquid to be heated.
A weir type submerged combustion device includes two concentric vertical tubes of relatively short length. The tubes are vertically displaced, with the top and bottom of the outer tube (the weir tube) being lower than the respective top and bottom of the inner tube (the combustion tube). This tube system is vertically mounted and is partially immersed in the medium; usually about half the vertical length of the tubes are immersed. The bottom of the outer tube is located above the bottom of the medium container. In operation, the combustion products (hot gases) are directed downward against the surface of the medium in the inner tube. The hot gases are forced to pass downwardly through the medium, around the lower edge of the inner tube and up through the medium in the space between the inner tube and the outer tube. The hot gases entrain the medium and force it to rise in the space between the inner and outer tubes and then over the upper edge of the outer tube. At this point, the partially cooled gases escape upward and out of the device, while the entrained, warmed water flows downward on the outside of the outer tube and into the medium container thus mixing with the remainder of the medium. The only agitation of the medium is that provided by the water flowing into the bottom of the weir and out over the top edge of the outer tube, and so additional agitation must be provided, at additional capital and operational cost.
The weir type of submerged combustion apparatus has a number of shortcomings, in that it fails to completely transfer heat from the combustion gases into the liquid to be heated and so is not optimally efficient. The weir type of submerged combustion fails to achieve complete agitation of the container, and thus does not provide sufficient agitation of the water to rapidly heat and evaporate the water deposited in the apparatus. In addition, the lack of complete agitation results in non-uniform temperature distribution in the water. Thus, additional agitation must be provided from an external mixing device. Without adequate agitation, a problem arises due to the non-uniform distribution of heat in the water. The weir type of submerged combustion is inefficient in transferring combustion heat to water because the close contact of the flame with the water forms water vapor in the combustion chamber. In addition, the full heat of combustion is exposed to only a small fraction of the total water. These effects combine to limit the water evaporation rate into the apparatus to less than what could be had with full agitation and complete and uniform heat transfer.
In the absence of added agitation by an external mixing device, the needed agitation of the water has been left to the often inadequate passive mixing resulting from the water flow through the weir. The lack of agitation directly results in a reduced rate of water evaporation and thus a loss in efficiency of fuel use as well. Other prior art systems include externally applied mechanical mixing devices, thus requiring the input of additional energy as well as provision of the additional equipment.
Due to these problems, such systems for water evaporation have not been widely used. Instead, collection in ponds for passive evaporation is often relied upon despite the adverse environmental effects.
The adverse environmental effects of such produced water have gained the attention of governmental agencies, and have increased the importance of finding economical alternative methods for disposing of such unwanted produced water. Complex systems such as that disclosed in U.S. Pat. No. 6,971,238 have been devised. While such systems may be good at disposing of the water in an environmentally acceptable way, these systems are expensive to build and expensive to operate, and cannot attain the maximum efficiency possibly obtained from the fuel used in the process.
An additional problem in the prior art has been the accumulation and concomitant requirement for removal of sand, mud, salt and other suspended and/or dissolved solids from systems such as those described above. To avoid accumulation of such materials, prior art systems have required that the incoming water be filtered prior to introduction into the system. Such filtering introduces an additional layer of complexity to the system, and adds to the capital and operational costs, as well as providing another source of possible operational interruption as a result of equipment breakdown or for required maintenance.
Removal of such debris is particularly problematical due to the wide variation in size and type of debris. For example, such debris may include sand, gravel, stones, wood, plastics of many shapes and sizes, finely divided organic and inorganic particulates and organic materials such as asphalts and crude oil, which are either inherently present in the water or which are inadvertently collected along with the water. In prior water disposal systems, the entire system must be shut down and the water drained out of the tank or other vessel in order to gain access to and remove the debris collected with the water, if it is not filtered out initially.
In addition to debris, water produced from hydrocarbon wells often includes various hydrocarbons in varying amounts. As the water is evaporated, these hydrocarbons tend to accumulate and must also be collected and disposed of properly.
Therefore, a need remains for an apparatus and method which will rapidly and efficiently dispose of such unwanted produced water and provide for an efficient debris and accumulated hydrocarbon removal apparatus in association with the water disposal.